The development of radio frequency microelectromechanical systems (RF MEMS) is largely motivated by possible applications in wireless communication devices. High quality, miniature and CMOS-technology compatible MEMS resonators are seen as an attractive alternative to existing RF passive components. In contemporary RF devices frequency-determining elements are implemented in two major ways. The first requires microfabrication of capacitors and inductors on-chip. Even though they consume the major part of the IC “real estate” and exhibit a low quality factor Q˜50, on-chip LC circuits are widely used for GHz-range devices.
Alternatively, quartz crystals, surface acoustic wave (SAW) filters, ceramic filters and high-Q LC are implemented as stand-alone elements and create a bottleneck for future miniaturization of RF devices. Their high quality factor (Q˜106 for quartz resonators and Q˜105 for ceramic SAW filters) plus high temperature stability, both indispensable features for wireless devices, force RF designers to accept these off-chip components. This approach conflicts with modem trends in wireless systems, especially those that require a radio-on-chip to be implemented with the smallest size and for the lowest price.
Micromechanical resonators as frequency-determining elements are expected to be the key for the radio-on-chip project. Modem surface micromachining tools allow fabrication of suspended silicon structures with a fundamental frequency of mechanical vibrations, f0, in the RF range. The MEMS resonator quality factor Q˜104 is a few order of magnitude superior compared to LC circuits. MEMS based signal generators and frequency standards may be incorporated into nearly any RF device.
The natural approach for building such a generator is to implement the MEMS resonator as a frequency determining element in the feedback loop of an amplifier and to provide conditions for self-sustained oscillations. The strain energy kx2/2, stored in a MEMS resonator can be comparable to electric energy LI2/2 in a conventional inductor.
However, methods to convert the electrical signal into mechanical motion of the resonator (drive) and back (detection) represent the major challenge in RF MEMS design. Piezoelectric transduction requires piezoelectric materials incompatible with CMOS technology. Magnetomotive methods for driving and detection used to demonstrate the highest resonant frequency in MEMS rely on high magnetic field and low temperatures.
It has been demonstrated that micromechanical reference oscillators can be operated using electrostatic drive and detection. However, capacitive methods are difficult to implement: relatively high voltage is required and narrow gap capacitors for both drive and detection impose significant restrictions on the design. Both oscillators employ beam-like geometry resulting in relatively low quality factor Q˜103. Problems related to cross-talk between drive and detection can be severe. The phase noise for either of these devices is significantly higher than one would expect from similar quartz-based oscillators and both groups attribute this extra phase noise to capacitive transduction problems. This motivates ongoing search for new design of micromechanical resonators and novel drive-detection methods.